Field of the Invention
This invention relates to the field of laser release materials for temporary wafer bonding.
Description of the Prior Art
Temporary wafer bonding (TWB) normally refers to a process for attaching a device wafer or microelectronic substrate to a carrier wafer or substrate by means of a polymeric bonding material. After bonding, the device wafer is thinned, typically to less than 50 μm, and then processed to create through-silicon vias (TSV), redistribution layers, bond pads, and other circuit features on its backside. The carrier wafer supports the fragile device wafer during the backside processing, which can entail repeated cycling between ambient temperature and high temperature (>250° C.), mechanical shocks from wafer handling and transfer steps, and strong mechanical forces, such as those imposed during wafer back-grinding processes used to thin the device wafer. When all of this processing has been completed, the device wafer is usually attached to a film frame and then separated, or debonded, from the carrier wafer and cleaned before further operations take place.
Most TWB processes use either one or two layers between the device wafer and the carrier wafer. In the case of a two-layer system, the first layer is a polymeric bonding material. It can be thermoplastic, thermosetting, or photocuring in nature. The polymeric bonding material layer is typically 10-120 μm thick and, more commonly, about 50-100 μm thick. The second layer is comparatively thin, and is present to enable facile separation of the bonded wafer pair after processing. The thin layer responds to radiation from a laser or other light source, which leads to decomposition of the layer itself or decomposition of the adjacent polymeric bonding material, causing bonding integrity to be lost within the structure and allowing it to come apart without applying mechanical force.
Laser-induced release is becoming a popular mode of debonding, and materials are available for operating at laser wavelengths ranging from the ultraviolet (e.g., 248 nm, 308 nm, and 355 nm) to the near infrared (e.g., 1064 nm). It should be noted that in some cases the polymeric bonding material has sufficient response to the laser radiation that a separate, thin light-sensitive layer is not required. However, in many, if not most, instances the use of a release layer greatly facilitates this process.
One problem with prior art laser release materials is that they require rather long curing times. For example, a curing time of one hour at 350° C. in a nitrogen environment is required to produce a film of Shin Etsu ODL-38 on the order of 1,000 Å thick. An even more complicated curing process is required for HD Microsystems HD3007, which is soft-baked at 80° C. and 120° C. and then cured at between 300° C. and 350° C. for an hour in nitrogen. These extremely long curing times significantly decrease the throughput of whole process.
There is a need for further materials suitable for use as a release layer that involve shorter cure times, while being readily susceptible to laser ablation.